Taking an argument to its supposed logical conclusion sometimes culminates in a travesty of logic. As George Eliot observed, only “the logic of a machine” is obliged to “go on to its last turn when it has been once wound up”. Is the school board being discriminatory for not hiring a teacher on account of her being x? Yes if “x” stands for “black” or “fat”; no if it stands for “paedophile ex-offender”. The logical “x” – intended to ensure objective consistency – is not always intersubstitutable.

Tell that to the moral philosopher Peter Singer! Extrapolating from racism and sexism, he argues that the way we automatically give humans precedence over other animals amounts to “speciesism”. Singer’s form of utilitarian ethics, which advocates the equal consideration and promotion of people’s interests (happiness being only one factor), insists that “chimpanzees are people, too” (the title of one of these 82 Brief Essays).

Given the essays’ brevity, and that they were originally journalistic pieces, perhaps he can be forgiven for not clarifying what he means by “interests”. He merely stipulates that humans, because they are able to think outside the present moment, usually have more of them, therefore greater entitlement to resources. But if, as he contends, a sense-deprived human may rightly be trumped by a monkey, why shouldn’t a sensitive slug be equally deserving? Another essay examines gradations of permissible euthanasia on babies with birth defects, although more cautiously than his 1993 paper that condoned infanticide up to 30 days after birth. Unsurprisingly, Singer’s public appearances are sometimes disrupted by disability rights campaigners and anti-eugenicists.

Also disdained by many fellow academics, Singer is nonetheless one of the most famous and widely read philosophers, and, as he here reminds us, was ranked among the first five of a German thinktank’s “100 Global Thought Leaders for 2013”. He apologises for the inevitable ephemerality of this rehashed journalism, but his topics (bioethics, global warming, vegetarianism, cloning, euthanasia, happiness) remain pertinent. His hard-line logic leads to intriguing sympathies (for the legalisation of incest and of doping in athletics) and to clear-eyed clarification, as in his even-handed support of free speech for both the Muhammad cartoonists and Holocaust deniers (his own parents had to flee Vienna in the 1930s).

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Often, however, it becomes “the logic of the machine”. Singer is a key proponent of “effective altruism”, which argues that charitable donation is “not morally optional” and each of us should donate a percentage of our income to charity (a tenth is recommended). It also seeks to ensure that we get the best return on our donations. Singer tots up the respective merits of contributing $100,000 to the prevention of trachoma or towards building a new museum wing. With the latter, you would be paying one-500th of the building’s cost and “could claim credit for the enhanced aesthetic experiences of 100,000 visitors. How does that compare with saving 1,000 people from 15 years blindness?” For Singer, it clearly doesn’t.

Felicific calculation and demands for unmitigated dutifulness, however, drain life of meaning. Estimating depression (“measured by how many years of good health it causes to be lost”) as the world’s fourth worst health problem, he ignores the fact that often depression’s source and symptoms are the sense of meaninglessness, which is precisely what the museum will help alleviate. Singer, it sometimes seems, knows the price of everything and the value of nothing.

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Jane O’Grady is visiting lecturer in philosophy of psychology at City, University of London, and co‑founder, London School of Philosophy.

Ethics in the Real World: 82 Brief Essays on Things That Matter
By Peter Singer
Princeton University Press, 376pp, £19.95
ISBN 9780691172477
Published 5 October 2016